One method for the recovery of liquid hydrocarbon present in the ground, where it floats atop the groundwater, involves the drilling of a well to below the water table. The method is described in U.S. Pat. No. 4,273,650 to Solomon, for example. A perforated well casing is inserted into the well, and water and hydrocarbon seep into the well through the perforated casing. By pumping water from the bottom of the well with a submersible pump, a cone-shaped depression in the water table occurs with the well as the center of the cone. The lighter hydrocarbon liquid collects in the well on top of the water, and is removed by a second, hydrocarbon, pump located at a fixed position near the liquid surface in the well.
The second pump has associated with it a point sensor that signals when the water/hydrocarbon interface is high, where the pump shuts off so that it will not pump out water instead of hydrocarbon. Another point sensor signals when the water/hydrocarbon interface is low, so that the second pump can operate, and pump out hydrocarbon.
Various improvements in this system have been suggested. Farmer, in U.S. Pat. No. 4,469,170, discloses a floating pump and sensor assembly. The floating assembly still includes only point sensors, so that the pump operates when a lower sensor is activated by the water/hydrocarbon interface, and shuts off when a higher sensor is activated by the rising water/hydrocarbon interface. Breslin, in U.S. Pat. No. 4,663,037, discloses a floating pump that relies on the specific gravity of the pump to float at an appropriate depth near the water/hydrocarbon interface, and has a hydrocarbon intake at the top of the pump, above the interface.
However, the optimum location of the intake of the hydrocarbon pump relative to the hydrocarbon/water interface varies at different stages of the operation of the system. When the depth of hydrocarbon above the water interface is great, the hydrocarbon pump intake is preferably a substantial distance above the interface, so that hydrocarbon instead of water is more certainly pumped out. When the depth of hydrocarbon above the water interface is shallow, the hydrocarbon pump inlet has to be very close to the water interface so that it can still pump out hydrocarbon.